1. Field of the Invention
The present invention relates to a resin composition composed mainly of polylactic acid resin, molded product thereof, and a method for their production.
2. Description of the Related Art
Related synthetic resins have been produced mostly from fossil resources such as petroleum, coal, and natural gas. They have a strong fear in their near future that fossil resources as their raw material will be exhausted soon, they accumulate in the natural world without decomposition after disposal, and their incineration emits carbon dioxide that causes global warming.
This notion has aroused a keen interest in biomass plastics which are produced from plants and microorganisms in place of fossil resources. Their raw materials are never exhausted because they originate from carbon dioxide in the atmospheric air through photosynthesis. In addition, biomass plastics are decomposed in the natural world and have less possibility of accumulating in the natural world without decomposition after disposal. Their incineration does not increase the concentration of carbon dioxide in the atmospheric air because they simply exist as carbon dioxide and are just temporarily used as biomass plastics, and then returned to the cycle of carbon in nature.
Among biomass plastics is polylactic acid resin, which is superior in processability and mechanical properties and available more easily and inexpensively than any other biomass plastics because it is being commercially produced. Polylactic acid resin finds use, on account of its biodegradability, as industrial materials, machine parts, medical care products, and such products as net, film, and sheet for agriculture, fishing, and civil engineering. It also finds use in applications which need impact strength.
Those which are used as a structural material for, say, enclosures of electric appliances need a high impact strength such that they hardly crack at room temperature (about 25° C.). The property which hardly cracks at about 25° C. means that the impact strength at about 25° C. is equivalent to an Izod impact strength or Charpy impact strength greater than about 10 kJ/m2. Those resins having such impact strengths usually have a glass transition point (Tg) no higher than 0° C. This will be described in the following.
The glass transition point is defined as the temperature at which polymeric substances, such as synthetic resins and natural rubber, which have a glass transition point, undergo glass transition. Any substance with a glass transition point has an amorphous moiety in which the intramolecular rotation of polymer chains is bound to frozen by intermolecular forces at a low temperature for molecular thermal motion to remain inactive. This state is called glass state. On the other hand, at a high temperature for active molecular motion, the intramolecular rotation of polymer chains overcomes the binding by the intermolecular force between polymer chains. This state is called rubber state. The glass transition point is a temperature in which the state transfers from glass state to rubber state. When the resin is heated to be higher than its glass transition point, any resin becomes capable of deformation but retains its original shape unless it is given a deforming force. This is the difference between glass transition point and melting point (or the temperature at which crystals melt).
Most of general purpose resins usually have a flexural modulus of elasticity equal to or greater than 1000 MPa at their temperature equal to or below their glass transition point; however, the flexural modulus of elasticity is lower than 1000 MPa at the temperature equal to or higher than their glass transition point. Eventually they assume a rubbery state, which is not suitable for use as a structural material for enclosures, for example.
Now, the polylactic acid resin can be used as a structural material for enclosures because it has a glass transition point of about 60° C. and its molded product has a flexural modulus of elasticity of about 2000 MPa at room temperature. However, it is as vulnerable to cracking as polystyrene because it is poor in impact resistance, with its Charpy impact strength being about 2 kJ/m2. It needs improvement in impact resistance if it is to be used as a structural material for enclosures of portable appliances.
As one way of improving polylactic acid resin in impact strength, polymer blending and polymer alloying to form a composite material composed of polylactic acid resin and another resin having higher impact strength than polylactic acid resin have been known. Resins suitable for combination with polylactic acid resin are those which have a flexural modulus of elasticity equal to or lower than 300 MPa at 30° C. Examples of such resins include polybutylene succinate, polybutylene succinate adipate, polybutylene terephthalate, polymer of aliphatic polyester of polylactic acid, polyamide, acrylonitrile-butadiene-styrene copolymer. Incidentally, any resin having a higher impact resistance than polylactic acid resin may be referred to as rubber henceforth in this specification.
In addition to the foregoing, the polylactic acid resin to be used as a structural material for enclosures of electric appliances should have an adequate modulus of elasticity at about 80° C. The polylactic acid resin is a polymer that takes on the crystalline structure. If any molded product of polylactic acid resin has a low ratio of crystallinity at room temperature, it will considerably soften and deform at temperatures above the glass transition point (about 60° C.) of the polylactic acid resin. It is known that the polylactic acid resin has an adequate modulus of elasticity at about 80° C. if it is crystallized and it has a certain degree of crystallinity. A possible way proposed so far of crystallizing the polylactic acid resin is by heat treatment during or after molding.
However, there is a problem that crystallization of polylactic acid resin takes a long time. In fact, polylactic acid resin takes a much longer time for crystallization than ordinary molding cycle, which is about one minute for injection molding. Therefore, if polylactic acid resin is to be crystallized completely in the mold, injection molding takes such a long time as to greatly reduce efficiency. In-mold crystallization is not practicable. Also, a longer cycle means that the polylactic acid which has been hot melted for the subsequent cycle in the cylinder of the injection molding machine experiences an extended heat history. This results in heat deterioration of polylactic acid resin and degradation of its molded products in mechanical properties, especially impact resistance. Moreover, crystallization of polylactic acid resin in the usual way gives rise to crystals of the order of microns to submicrons in size, and such large crystals scatter light, making polylactic acid resin white turbid and opaque.
In order to address this problem or in order to promote crystallization, attention has been turned to the addition of a so-called nucleating agent. The crystal nucleating agent becomes the primary crystal nucleus of a crystalline polymer and promotes the crystal growth of a crystalline polymer. In a broad sense, it includes one which promotes crystallization of a crystalline polymer. It also includes one which accelerates the rate of crystallization of a polymer. The former crystal nucleating agent makes fine the crystals of a polymer, thereby improving the polymer's stiffness and clarity. Both of the crystal nucleating agents can accelerate the rate of crystallization, thereby reducing time required for crystallization and also reducing the molding cycle time if molding and crystallization proceed simultaneously.
Examples of the efficient nucleating agent for polylactic acid resin include a metal salt of phosphonic acid having an aromatic ring (see Japanese Patent Laid-open No. 2006-89587 (claims etc.)) and a salt of melamine compound (see Japanese Patent Laid-open No. 2005-272679 (claims etc.)). Additional examples include polycyclic pigments and azo pigments (see Japanese Patent No. 4019414 (claims etc.), International Publication Nos. WO2004/022649 (claims etc.) and WO2004/069932 (claims etc.), and Japanese Patent Laid-open Nos. 2005-264147 (claims etc.) and 2006-307036 (claims etc.)).